Tumble drying processes are a mainstay of both domestic and industrial textile fabric cleaning procedures and typically involve placing the textiles in a container such as a cylindrical drum which is rotated in alternating clockwise and anti-clockwise cycles whilst hot air is introduced into the drum. Domestic dryers typically comprise cylindrical drums having solid walls and the hot air is introduced at the rear of the drum, whilst the cylindrical drums in industrial dryers may have perforated side walls, such that the hot air may enter through the perforations. A combination of the hot air treatment and the mechanical action of the tumbling process causes water to be expelled from the textile materials in order that drying is achieved.
However, such processes, though generally very effective, are usually characterised by high levels of energy consumption, both in terms of effecting rotation of the container and, most particularly, in generating heated air. Typically, prior art processes may involve prolonged treatments at high temperatures in order to effect the required degree of drying. Clearly, however, the lower are the energy requirements of a system, the more efficient is the system and its associated drying process. Consequently, there is a desire to reduce both the time of such drying treatments and the temperature at which they are carried out in order to provide more efficient processes, whilst maintaining equivalent drying performance.
Current efficient domestic tumble dryers are graded in terms of energy consumption according to EU Directive 92/75/EEC and, more specifically, Directive 95/13/EEC, with category ‘A’ dryers being the most efficient, and category ‘G’ the least efficient. Hereinafter, energy consumptions are quoted for the cotton drying cycle for each machine type, in kWh/kg of drying load. Thus, for vented tumble dryers, ‘A’ class consumption is <0.51 kWh/kg, ‘C’ class (most common) is between 0.59 and 0.67 kWh/kg, whilst ‘G’ class is >0.91 kWh/kg. These values differ slightly for condenser tumble dryers, with ‘A’ class at <0.55 kWh/kg, ‘C’ class (most common) at between 0.64 and 0.73 kWh/kg, and ‘G’ class at >1.00 kWh/kg. With average domestic dryer capacities now at around 8.0 kg, this equates to a typical consumption for a ‘C’ class vented tumble dryer of 4.7-5.4 kWh/cycle; an ‘A’ class equivalent machine would run at <4.1 kWh/cycle. The most recent system in the EU (arising from Commission Delegated Regulation 392/2012, which entered into force on 29 May 2012 and will start applying from 29 May 2012) has, however, seen a switch to a new rating system for domestic tumble driers. This considers annualised energy consumption and derives an energy efficiency index (EEI), as well as introducing three new classes on top of class A, these being A+, A++ and A+++(most efficient). An EEI value of <24 results in an A+++ energy efficiency rating. Performance levels in the domestic sector generally set the highest standard for an efficient fabric drying process. Energy consumption in industrial tumble drying is usually higher, due to the need for faster cycle times. It is also noteworthy that, overall, tumble drying is significantly less efficient than washing as a component part of the laundry process in either sector.
Heating of the circulating air is the principal use of energy in such tumble dryers and the present inventors have therefore sought to effect improvements in the prior art processes by reducing the temperature levels required in such processes. This has been possible by means of changes made to the mechanical action of the process on the fabric in the drying load. Mechanical action in a conventional, horizontal axis tumble dryer is generated by the forces acting on the fabric through falling and hitting either other fabric or the dryer inner drum surface, whilst the fabric is interacting with the forced hot air flow. This results in release and evaporation of water from within the fabric, and hence drying. In the method herein provided, alteration of the mechanical action of the process in order to promote more localised release and evaporation of water at the fabric surface has resulted in lower drying temperatures. As a further potential benefit, it has been found that the changes made can also reduce the degree of fabric folding, and hence the level of creasing associated with tumble drying. Creasing, which concentrates stresses during this drying process, is a major source of localised fabric damage. Ironing at high temperatures is then the conventional means used to remove such creasing and this, too, brings a fabric damage penalty. Prevention of fabric damage (i.e. fabric care) is of primary concern to the domestic consumer and the industrial user. Furthermore, if creasing is reduced, there is also the secondary benefit to the user of convenience resulting from less ironing.
Hence, the present inventors sought to devise a new approach to the drying problem, which allows the above deficiencies associated with the methods of the prior art to be overcome and thereby provided a method which eliminates the requirement for the use of high drying temperatures for extended periods of time, but is still capable of providing an efficient means of water removal, so yielding economic and environmental benefits. The method also promotes fabric care through reduced creasing and fewer requirements for subsequent ironing.
Previously, in WO-A-2007/128962 there was disclosed a method and formulation for cleaning a soiled substrate, the method comprising the treatment of the moistened substrate with a formulation comprising a multiplicity of polymeric particles, wherein the formulation is free of organic solvents. In preferred embodiments, the substrate comprises a textile fibre and the polymeric particles may, for example, comprise particles of polyamides, polyesters, polyalkenes, polyurethanes or their copolymers, but are most preferably in the form of nylon particles.
The method disclosed in this document has been highly successful in providing an efficient means of cleaning and stain removal which also yields significant economic and environmental benefits due to its use of a cleaning formulation which requires the use of only limited amounts of water. The present inventors therefore sought to provide a drying process which adopts a similar approach to that disclosed in WO-A-2007/128962, and which offers benefits in terms of reduced energy requirements, whilst still providing an acceptable level of performance, and succeeded in achieving at least equivalent drying performance whilst employing significantly reduced process temperatures.
Thus, in WO-A-2012/098408 a process is provided wherein the drying effect achieved as a consequence of mechanical interaction of a wet substrate with physical media is optimised, such that excellent drying performance may be achieved at much lower temperatures (i.e. low energy) without extending drying times. Additional benefits have also been observed in terms of the reduction of fabric creasing and associated fabric damage. Specifically, there is provided a method for the drying of a wet substrate, said method comprising treating the substrate with a solid particulate material at ambient or elevated temperature, said treatment being carried out in an apparatus comprising a drum comprising perforated side walls, wherein said drum comprising perforated side walls is rotated so as to facilitate increased mechanical action between said substrate and said particulate material.
The method of WO-A-2012/098408 derives from an appreciation on the part of the inventors that optimum drying performance can be achieved as a result of improved mechanical interaction between substrate and physical media. This can be effected by the use of solid particles in the drying process and is a function of the number, size and mass of the particles and the free volume within the vessel in which the drying operation takes place, in addition to the G force dictated by its speed of rotation. Free volume in this context refers to the space inside the vessel which remains unoccupied by wet substrate or particulate media, and G force is defined on the basis of the centripetal forces which are acting.
Even though WO-A-2012/098408 describes a method which can dry a substrate effectively, the present inventors have now sought to provide an apparatus and method offering further improvements. In particular, the present invention attempts to solve, at least in part, one or more of the following problems: (i) removal and collection of the solid particulate material, (ii) improved drying efficiency, (iii) improved fabric care and (iv) reduced fabric creasing.